The present invention relates to a cooling apparatus and a process of manufacture of plastic film or sheet obtained by blowing tubing.
An apparatus is already known which is intended to produce blown tubular film or sheet of good clarity and which is constituted by an assembly of three juxtaposed elements: following the path of the tubing, this includes a closed chamber, for reheating, at the lowermost section of the apparatus, then in its intermediate section an intensive blowing ring, the air flow of which is directed perpendicularly to the tubing, this ring having the function of accelerating the solidification of the material, and finally in its upper section an upper chamber in which the tubing is in contact with an air flow directed in the drawing direction of the tubing.
Such an apparatus has already been described in the European Patent Application published under the number 0 041 803. This apparatus enables polyethylene sheets to be extruded at an expansion ratio equal to 2. However, it has the disadvantage of not permitting satisfactory extrusion of polyethylene sheets at an expansion ratio greater than 2, as is demonstrated in the comparative examples below. Besides this, such an apparatus has the disadvantage of only being able to improve the optical properties of the tubing for resins such as free-radical low density polyethylene, ethylene/vinyl acetate copolymer, and linear low density polyethylene, the optical properties of which were already relatively satisfactory. By satisfactory optical properties for application to the manufacture of tubing is meant a turbidity (determined according to ASTM D 103-77) not greater than 7%. Finally, a last disadvantage of this apparatus resides in the impossibility of improving the optical properties of the tubing (with respect to a conventional cooling system) while insuring a high throughput of material.